Admixtures for concrete are used in order to increase the strength of the concrete and include additive types for entraining air in concrete; or for water reduction; or plasticizing admixtures or dispersion agents. To the last mentioned group are also counted plasticizing agents and superplastiziers or liquidizing agents.
Air is entrained in the liquid mass as a result of the mixing of the fresh concrete irrespective of the presence of an air pore forming agent. By stirring, air is drawn into the material when it is moving from the periphery of the mixer towards the center. As a result of the whirling motions of the mixer, turbulence in the mass causes air to be entrained at the surfaces where shearing occurs, causing enclosure and entrainment. The forming, dispersion and stabilization of the air pores is a fundamental process in the forming of emulsions in general.
On the other hand, by intense stirring and especially when turbulence occurs, some small desireable pores will coalesce so that larger pores or voids will be formed. Such a coalescence is natural as it provides a reduction of the surfaces and is directed towards decreasing of the free energy in the system.
The principal function of air-entraining agents is to hinder coalescence during mixing, transport, wetting and consolidation. This can be obtained if the walls of the pores, by adsorption of this agent at the air-water interface, form a deformable film with a polar surface, sometimes with an electrostatic charge, towards water. The air pore stabilizing mechanism is based on the fact that the molecules of air pore-forming agents have separated hydrophilic and hydrophobic portions. They are thus surface-active substances, which will be adsorbed at the air-water interface with the hydrophobic portion directed inwards towards the air, and the hydrophilic portion adsorbed in the water surface. Such an interfacial film will lower the surface tension of the water. By this effect large air pores can be sub-divided into smaller pores by the shearing action of mixing.
However, the diffusion of air from the pores at the water surface also has an importance for the distribution and control of pore dimensions in the pore system. The air pressure within the pores is reciprocally proportional to the pore diameter. The smaller the diameter, the higher the air pressure inside the pores. As higher pressure causes an increased concentration difference, the diffusion velocity is increased, whereby small pores disappear rapidly and larger pores increase in dimension. This diffusion effect results in a coarser air pore system. This will reduce the frost-resistance of the hardened concrete.
Frost-cracking resistance of concrete is usually based on the determination of the "spacing factor". This in general, is a term for the closest distance from a point in the solid material to the nearest pore surface.
Upon freezing, water increases about nine per cent in volume. If there are insufficient air pores of at least equal volume to the increased volume of the water upon freezing, the water can crack the concrete by the formation of ice. Upon freezing, the water expands in narrow capillary channels in the concrete matrix. The pressure of the water increases according to the distance to the surface of the air pores and if this distance is too large, the concrete will crack. This spacing factor, for a constant volume of the air pores, will be smaller, when the dimension of the air pores is smaller.
It is also important to have the total air pore volume as small as possible. In general an increase of the air pore volume by one per cent will cause a decrease of the compressive strength by about five per cent. It is therefore of great importance to obtain a stable air pore system with the smallest possible pore volume. The result will be the lowest, least possible reduction of strength of the concrete.
The capillary water suction and the risk of reaching the critical degree of water saturation is decreased by introducing an air pore system. The transport area for the water between the air pores will be decreased and the transport distance will be increased.
By introducing air pores, a further, important effect will result in modification of the structural formation of the material between the air pores. The water concentration is higher in the layer nearest to the pore wall than at a distance from the pore in the concrete. By hardening of the matrix between the air pores, the concrete will be stronger. The lower porosity gives a lower capillary absorption velocity as the capillary channels have smaller dimensions. This is a further contributing factor to slowing capillary suction rates, especially in systems with smaller air pores.
The existence of an air pore system which is optimal in its geometrical structure is necessary but is often not sufficient to reach good frost-resistance properties. Air pore systems with pore walls, that are not water repelling, will not give the concrete as high resistance against frost-cracking as an air pore system with pore walls that are hydrophobic. The function of air pore-forming agents will result in an orientation of the hydrophobic ends of the molecules towards the air-phase within the pores. When the hydration has started, the surface of the air pore can more or less be covered by hydration products which are hydrophilic. Macro-molecular surface-active substances and polymer particles can retain a part of the hydrophobic character without reduction of the same by the hydration products contrary to small surface-active molecules, for example metal soap molecules. Our invention has in that respect unique properties, so that the wall of the air pores remain hydrophobic after hydration of the cement.
It is well known that concrete which contains sand with particles smaller than 0.125 mm and with a marked low content of particles in the range 0.2-0.6 mm will have a pore system which is not effective against frost-cracking. In such cases it is not possible to obtain a sufficient air pore volume and the desired size distribution of the pores. Better results are obtained when the filler part is replaced by particles of the 0.2-0.6 mm fraction.
The group of substances on which the present invention is based are certain water-insoluble, high-molecular weight cereal storage proteins as the main admixture in controlled entraining of air in concrete.
In U.S. Pat. No. 2,521,073, Ludwig et al, it is suggested to use a number of proteins containing materials, among them wheat flour as an admixture to Portland cement. Also the use of gluten derived from wheat flour is suggested as an air-entraining agent for light weight concrete. It is obvious that the possibilities in the hardened concrete and particularly the ability of frost-resistance performance of gluten with combined flow properties, were not realized by Ludwig et al.
These cereal storage proteins have unique surface active chemical qualities. The best known example of these proteins is gluten from wheat, which gives a gel with water. These substances eliminates many of the disadvantages of other used pore-forming agents. By entrapping air during the mixing procedure, it does not matter which fraction size of the aggregate particles in the mix or concrete is chosen. This is demonstrated by mixing cement paste only, i.e. cement and water, with the proposed admixture. An air pore system of predetermined volume and the correct pore size distribution is obtained. Utilizing the same mixer, the same desireable airpore system is not obtained if other, prior art air pore agents are used instead. Further, the air pore-forming agent stabilizes the air pores and fixes the pores in the cement paste, by strengthening of the pore walls due to film formation and by preventing air diffusion between the pores.
Even if gluten has superior properties compared to other, air pore forming agents, such as derivates from petroleum products, there are no reports of large-scale use of gluten. The reason is certainly that gluten as a biological material has variable properties depending on variations in the genetic species used for the production of the corresponding wheat. There are also variations due to climate and the earth. The influence of these factors on the properties of the gluten results, as we have observed, in large variations in the properties of the concrete. Thus we lack reproducibility, which can not be accepted in industrial use.
The object of the invention is to eliminate the mentioned disadvantages connected to the prior use of gluten as an air-entraining agent by providing a method for the production of a compound usable by the production of frost-resistance of concrete so that the same will have a reproducable effect and will give controlable properties to the concrete, and that predetermined properties can be obtained with good accuracy in industrial scale production.
A further object is to provide a method to produce an on cereal storage proteins compound working as an air-entraining agent by concrete production which compound as an admixture is compatible with superplastizers without disadvantageous effects as by prior art admixtures.